1. Field of the Invention
The present invention relates to a radio communication device, a radio communication system and a measurement method, and more particularly to a radio communication device, a radio communication system and a measurement method capable of performing communication control as represented by transmit power control of signals (frames) based on communication quality.
2. Description of the Background Art
A radio communication device, such as a portable phone, a PHS (Personal Handyphone System), a note type personal computer provided with a wireless LAN (Local Area Network) function or the like, operates with a battery as its power source. To prolong usage of the radio communication device, reduction of consumed power is an important issue. In order to decrease power consumption, it is particularly effective to restrict transmit power.
For the purpose of restricting the transmit power, a control method as follows, for example, has been employed. A radio communication device on the receiving side measures received field strength based on radiowaves transmitted from a radio communication device on the transmitting side, and transmits information of the received field strength measured to the radio communication device on the transmitting side. The radio communication device on the transmitting side refers to the information of received field strength received from the radio communication device on the receiving side, and transmits data by setting transmit power to a lower value within the range not impairing the communication.
There are IEEE standards 802.11 among wireless LAN standards. IEEE 802.11 includes standards defined by several task groups. A standard defined by a major task group is called IEEE 802.11h, which defines the standard for operation in the 5 GHz band in Europe. IEEE 802.11h defines TPC (Transmit Power Control) and DFS (Dynamic Frequency Selection) for reducing power consumption in a radio communication device. TPC is a function of controlling transmit power in accordance with a condition such as a distance between a STA (STAtion) that is a terminal station identified as a radio communication device and an AP (Access Point) that is a control station as will be described below. DFS is a function of dynamically changing radio channels in accordance with their qualities and automatically selecting a radio channel free of interference or the like. Such techniques are disclosed in “IEEE Std 802.11h-2003”.
IEEE 802.11e is a standard defined by another task group. In the draft of IEEE 802.11e, QoS (Quality of Service) guaranteed communication, DLS (Direct Link Setting) mechanism and others are defined, while maintaining compatibility with MAC (Medium Access Control) standards of IEEE 802.11. QoS is a technique of guaranteeing a constant communication speed by securing a band for transmitting streams and others over the network. DLS is a mechanism for allowing direct communication between STAs, which are terminal stations identified as the radio communication devices, without intervention of an AP. Such techniques are disclosed in “IEEE P802.11e/D13.0”.
Hereinafter, a procedure where a STA (radio communication device) 1102 uses DLS to perform TPC will be described with reference to FIGS. 45-49. FIG. 45 schematically shows an infrastructure network configured with a single BSS (Basic Service Set) 1110. BSS 1110 may include, for example, an AP 1101, which is a radio communication device set as a control station, and STA 1102 and STA1103, which are radio communication devices. Herein, it is assumed that STA 1102 is the radio communication device on the transmitting side (transmitting station) that transmits frames originally intended to be transmitted, and STA 1103 is the radio communication device on the receiving side (receiving station) that receives the frames originally intended to be transmitted which have been transmitted from STA 1102.
FIG. 46 shows the processing flow where STA 1102 on the transmitting side transmits frames by controlling transmit power. Firstly, STA 1102 on the transmitting side transmits a frame (Measurement Request Frame) 3900 to instruct STA 1103 on the receiving side to measure quality of a radio channel (hereinafter, also referred to as “communication quality”) (step S (hereinafter, abbreviated as “S”) 3800).
Here, frame (Measurement Request Frame) 3900 is described. FIG. 47 shows a frame format of a part of frame (Measurement Request Frame) 3900. Frame (Measurement Request Frame) 3900 has the part where measurement conditions are designated in measurement type (Measurement Type) field 3901, a measurement channel (Channel Number) field 3902, a measurement start time (Measurement Start Time) field 3903, and a measurement duration (Measurement Duration) field 3904.
Referring again to FIG. 46, STA 1103 on the receiving side having received frame (Measurement Request Frame) 3900 performs measurement of communication quality in accordance with the measurement conditions designated in frame (Measurement Request Frame) 3900 (S3805). STA 1103 measures received field strength, for example, as the communication quality. Thereafter, STA 1103 on the receiving side transmits to STA 1102 a frame (Measurement Report Frame) 4000 showing information of communication quality (hereinafter, also referred to as “communication quality information”) that is the measurement result (S3810).
Here, frame (Measurement Report Frame) 4000 is described. FIG. 48 shows a frame format of a part of frame (Measurement Report Frame) 4000. FIG. 49 shows definition of RPI (Receive Power Indicator) histogram information for each received level. Frame (Measurement Report Frame) 4000 has the part where a measurement type (Measurement Type) field 4001, a measurement channel (Channel Number) field 4002, a measurement start time (Measurement Start Time) field 4003, and a measurement duration (Measurement Duration) field 4004 are specified. When measurement type field 4001 specifies the RPI histogram information as shown in FIG. 49, time density histogram information for respective received levels (PRI0 density 4010 to PRI7 density 4017) as shown in FIG. 48 is recorded on frame 4000. Herein, stream data of which RPI is K (natural number) is represented as RPIK.
STA 1102 controls transmit power of the frames to be transmitted to STA 1103 based on the time density histogram information of each received level recorded on the received frame (Measurement Report Frame) 4000, and transmits the frames originally intended to be transmitted (S3815).
The above-described control method for restricting transmit power, however, has the following problems. The communication protocol of IEEE 802.11 adapts a CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) method. CSMA/CA is means for avoiding contention when a plurality of STAs access one and the same channel. Specifically, in order to permit equal communication opportunities to all the STAs and to avoid interference, it is configured such that a STA refrains from transmitting radiowaves while detecting radiowaves from another STA.
When not detecting radiowaves from another STA over a prescribed time period, the STA transmits radiowaves after a lapse of random back-off time. That is, during the time when STA 1103 is performing measurement of communication quality, the radiowaves transmitted from AP 1101 or another STA other than STA 1102 cannot be rejected completely. This means that the communication quality information measured by STA 1103 so as to enable STA 1102 to perform transmit power control may not be effective. In such a case, STA 1102 cannot perform proper control of transmit power based on the communication quality information recorded on frame (Measurement Report Frame) 4000.
Meanwhile, in the draft of IEEE 802.11e, HCCA (HCF (Hybrid Coordination Function) Controlled Channel Access) is defined as a mechanism for conducting QoS guaranteed communication. In HCCA, a HC (Hybrid Coordinator) assigns to a STA a right (hereinafter, referred to as “communication right”) permitting transmission of frames over a prescribed period, and the STA assigned with the communication right can transmit frames to another STA over the prescribed period.
The function as the HC performing control of assignment of the communication right is also carried out by AP 1101. STA 1102 does not know in which timing and how long the communication right will be assigned by AP 1101, and thus, it cannot transmit frame (Measurement Request Frame) 3900 to STA 1103 to cause it to measure communication quality by rejecting the radiowaves transmitted from another STA.
Even in the case where STA 1102 somehow knows the timing and period for assignment of the communication right and transmits frame (Measurement Request Frame) 3900 for measurement of communication quality, there may be a case where the transmitting station, STA 1102, transmits the frame not only to STA 1103 but also to another STA. In such a case, the transmitting station, STA 1102, will have to transmit frame (Measurement Request Frame) 3900 to a plurality of STAs for conducting the control of transmit power. In order to transmit frame (Measurement Request Frame) 3900 to a plurality of STAs, the transmitting station, STA 1102, needs to calculate and adjust the timings for transmitting frame (Measurement Request Frame) 3900, the measurement start times, the measurement durations and others. This complicates the mechanism of the transmitting station, STA 1102, and accordingly, the device cost for implementing the transmit power control will increase.
In IEEE 802.11h, the mechanism called Quiet prohibiting transmission of radiowaves over a designated channel for a certain period is defined. With Quiet, however, all the STAs in BSS 1110 are prohibited from transmitting radiowaves, which cannot solve the above-described problems.